A stroboscope is a common tool for “freezing” the apparent motion e.g. of a vibration object. It produces the optical effect of stopping or slowing down an object to allow you to observe and analyze the object's motion.
Stroboscopic illumination has been applied in a variety of instruments, including interferometers (see, for example, Harris, J. S., Fusek, R. L., Dayton, University, Dayton, Ohio), AB; Marcheski, J. S., Dayton, University, Dayton, Ohio “Stroboscopic interferometer” Applied Optics, vol. 18, Jul. 15, 1979, p. 2368–2371 (1979); Kwon, Osuk Y., Shough, Dean M., Lockheed Missiles and Space Co., Inc., Research and Development Div., Palo Alto, Calif.), AB; Williams, Rick A., Lockheed Missiles and Space Co., Inc., Research and Development Div., Palo Alto, Calif. “Stroboscopic phase-shifting interferometry” Optics Letters (ISSN 0146–9592), vol. 12, p. 855–857 (1987); Marshall, Gerald F., Consultant in Optics; Shough, Dean, Kwon, Osuk Y., Lockheed Research and Development Div.; Ketabchi, Mehrdad, Vettese, Thomas, Speedring Systems, Inc. “Stroboscopic interferometry of high-speed scanning mirrors” Proc. SPIE Vol. 1987, p. 212–220, Recording Systems: High-Resolution Cameras and Recording Devices and Laser Scanning and Recording Systems, Leo Beiser; Reimar K. Lenz; Eds. (1993); Ken NAKANO, Kazuhiro HANE, Shigeru OKUMA and Tadashi EGUCHI “Visualization of high-frequency surface acoustic wave propagation using stroboscopic phase-shift interferometry (Paper #: 3225-05) SPIE Proceedings Vol. 3225 Microlithography and Metrology in Micromachining III ISBN: 0-8194-2657-1, 142 pages (1997); and Nakano K., Hane K., Okuma S., Eguchi T., “Visualization of 50 MHz surface acoustic wave propagation using stroboscopic phase-shift interferometry,” Optical Review, vol. 4, number 2, p.265–269 (1997)). The most recent applications include vibrating MEMS devices, for which a stroboscopic or pulsating light source allows one to apply any one of several known surface profiling techniques based on interferometry, for example phase shifting interferometry.
One potential complication in applying stroboscopic illumination to an imaging interferometer is wavelength instability. Most interferometric systems depend on a stable wavelength, and may fail if the mean wavelength or other spectral properties of the source vary because of changes in environmental conditions or adjustments in illumination strength. In coherence-scanning interferometers, the fringe contrast envelope often must be assumed to be of a particular functional form, such as Gaussian, in order to be accurate; and distortions of this envelope shape due to surface colors or unexpected or unusual source spectra can also lead to significant errors. For many candidate sources, such as LED's, the wavelength can drift significantly with temperature and operating current. The wavelength of a pulsed LED can be particularly unstable and dependent on duty cycle. In a conventional interferometer, a few nm change in effective wavelength on a sample having 100 mm steps could lead to several hundred nanometers of error. As Novak has observed (see Erik Novak, Der-Shen Wan, Paul Unruh, Joanna Schmit, “Dynamic MEMS Measurement Using a Strobed Interferometric System with Combined Coherence Sensing and Phase Information,” Proceedings of American Society of Precision Engineering Winter Topical Meeting on Machines and Processes for Micro-scale and Meso-scale Fabrication, Metrology and Assembly, 104–107 (2003)), this is significantly higher than the noise floor of most optical profilers, which is generally a few nanometers or less. Combined phase and coherence sensing techniques are often more sensitive to wavelength errors, with print-through of the interference fringes when incorrect wavelengths are used.
Accordingly, it is a primary object of the present invention to provide a stroboscopic interferometric microscope system for measuring microscopic structures.
It is another object of this invention to provide a stroboscopic interferometric microscope system for measuring microscopic structures while being insensitive to changes in the wavelength of the stroboscopic pulses illuminating the structures.
It is yet another object of this invention to provide a stroboscopic interferometric microscope system for measuring microscopic structures using frequency-domain analysis (FDA).
Other objects of the invention will in part be obvious and in part will appear hereinafter when the following detailed description is read in connection with the accompanying drawings.